Inserts of the above kind for steel drums conventionally have also been made of steel, for reception of a pressed steel plug. A thread is rolled on the outside of the plug, for co-operation with the insert internal thread. The insert is typically fitted to a drum as follows. The drum maker forms a drum head with an aperture surrounded by an upset, outwardly projecting collar. The insert is inserted through the collar and has a radially outwardly projecting flange at its inner end which butts up against the inside of the drum head. In this position, a tubular outer end of the insert projects outwardly beyond the drum head collar. The projecting tubular outer end is then curled outwardly and downwardly around the circumference of the collar, so as to trap the insert permanently in position. Prior to insertion, one or more sealing washers are slipped over the insert so as to rest against the radially projecting flange. During the curling operation, these washers are forced into the gap between the insert and collar, to form a permanent, fluid-tight seal. At the filling line, after the drum has been filled, the plug, equipped with another suitable sealing washer, is screwed into the insert to seal in the contents. The end user may then unscrew and reclose the closure plug as often as they need to.
For contents which are corrosive or sensitive to contamination (e.g. medical or food grade products, or paints) the drum interior, insert and plug must be suitably lacquered to prevent such corrosion or contamination. With repeated removal/replacement of the plug, the lacquer, particularly in the thread area of the plug and insert, can become damaged, exposing the container contents to bare metal and hence causing corrosion/contents contamination problems. With volatile contents, all-metal drums, inserts and closure plugs pose a risk of explosion if the drum is exposed to very high temperatures, e.g. in a fire. Plastics closure plugs are prescribed for use in these situations. They will melt to provide pressure relief in case of fire, and are corrosion resistant. Plastics materials are also available in food/medical grades. But they do not solve the problem of exposed metal on a worn or damaged insert. Nor is there any guarantee that the drum filler and (to a greater extent) the end user, will not incorrectly replace the plastics plug with a metal one. These problems are mitigated by providing a plug-receiving threaded insert made from plastics material. This will fuse in the event of fire and can be made inherently corrosion resistant/non-contaminating, in the same way as a plastics plug. As the insert is permanently installed in the drum, it cannot readily be replaced with a component of the incorrect type. Special threadforms can be used (e.g. a buttress thread) to ensure compatibility only with plastics plugs. On the other hand there is design freedom to provide generic threadforms compatible with either plastics or metal plugs, where universal use/reduced parts inventories are a customer requirement.
U.S. Pat. No. 4,195,750 (Fee) and U.S. Pat. No. 4,588,103 (Baughman) disclose an internally threaded plastics insert (“flange”) used to receive a complementary threaded plug which forms a fluid-tight sealed closure for a container. An upstanding boss surrounding an opening is formed from the sheet metal of the container wall. The plastics insert is fitted into the boss from inside the container, with a portion of the boss overlying at least an upper face of the insert. The boss is then crimped around the insert to engage serrations and a circumferential groove formed around the outside of the insert, to retain the insert non-rotatably in the container opening. A plug may then be screwed into the insert, with a sealing washer which engages the portion of the boss overlying the insert upper face. Therefore no further sealing washer is required between the insert and the material of the container. A resilient washer may be provided between a flange on the inner end of the insert and the adjacent container sheet material surrounding the boss; but the primary purpose of this washer is to trap any debris (e.g. lacquer flakes) dislodged during the crimping operation and prevent it from falling into the container. Although the disclosed plastics insert has several advantages, it requires relatively complex specialist tooling to form the boss and perform the crimping operation. An exposed edge of the container sheet metal can remain accessible by the container contents via the plug/insert threads. If the crimp process is not properly controlled or tooling wears down or the material of the flange creeps over time, then the insert can rotate in the drum causing a serious issue which is typically not discovered until the drum is filled and the plugs are final torqued, or during removal of the plug by the end user. A more conservative plastics insert design, which in some aspects is closer to well-proven and reliable metal insert designs and which is easy for the drum maker to fit, is therefore desirable.